1. Field of the Invention
The present invention relates to a communication apparatus which performs facsimile communication on an IP network by using a VoIP (Voice over IP) technique, a control method thereof, and a storage medium.
2. Description of the Related Art
Recently, a technique for performing facsimile (FAX) communication on an IP network is being standardized. An example of such a standardized scheme is the T.38 scheme in which a G3 FAX (T.30) signal is converted into an IP packet by using a protocol called IFP (Internet Facsimile Protocol), and the IP packet is transmitted in real time on an IP network. There is also a deemed voice scheme in which a G3 FAX (T.30) signal is digitized by using the VoIP (Voice over IP) technique, and the digital signal is transmitted in real time as a voice signal on an IP network.
The VoIP technique used in FAX communication by the deemed voice scheme is a technique of sampling an analog voice signal, converting it into a digital voice signal, compressing the digital voice signal by voice encoding, further converting the compressed signal into an IP packet, and transmitting the IP packet to an IP network. Generally, the VoIP technique often adopts a lossy compression voice encoding scheme to increase the compression ratio. This is premised on that, when a loss of data occurs in a voice signal on a transmission path, the voice can be recognized by the human auditory sense as long as degradation of the voice quality is limited to a certain degree, unlike a data signal. For example, in an IP-PBX environment, voice encoding schemes such as G.729 (CELP, 8 kbps), G.726 (ADPCM, 32 kbps), and G.711 (PCM, 64 kbps) having different communication rates (transmission bandwidths) depending on the compression ratio are employed.
Japanese Patent Laid-Open No. 2008-228219 has proposed a technique in which the above-described voice encoding scheme is used in control of facsimile communication. A data communication terminal described in this reference decides a communication rate in T.38 facsimile communication based on the voice encoding scheme. More specifically, the data communication terminal performs a voice call connection to execute voice communication with a partner terminal connected via an IP network, and then performs a T.38 call connection to execute facsimile communication. At this time, the data communication terminal performs T.38 facsimile communication at a communication rate decided based on the type of voice encoding scheme (voice codec) used in voice communication.
When transmitting a G3 FAX signal by the deemed voice scheme, the G3 FAX signal is deemed as a voice signal, voice-encoded, and converted into an IP packet, as described above. In this deemed voice scheme, when a scheme other than the G.711 non-compression scheme is used as the voice encoding scheme, a communication terminal on the receiving side cannot accurately reproduce a G3 FAX signal used before voice encoding (compression). For this reason, the communication terminal on the receiving side cannot accurately reproduce a control signal and image signal for G3 FAX communication, and a communication error may be generated. For example, in a high-communication-rate modulation scheme such as V.34 out of modem modulation schemes supported by G3 FAX, a communication error may be generated depending on the compression ratio of the voice encoding scheme.
Conventionally, a communication terminal which performs FAX communication by the deemed voice scheme includes only an analog line interface for a public telephone network, and utilizes a dedicated gateway or adaptor to use the VoIP function. That is, the communication terminal itself does not have an interface corresponding to an IP network, and cannot recognize a voice encoding scheme used in VoIP. Hence, such a communication terminal cannot perform communication control in accordance with a voice encoding scheme to be used, in order to cope with degradation of the quality of a voice signal corresponding to a G3 FAX signal.